Usually, wind turbines run at almost constant rotational speed with direct grid connection. However, since the amount of wind energy, i.e. the aerodynamic conditions may vary, indirect grid connections are known, whereby the wind turbine generator runs in its own, separate alternating current (AC) grid. This grid is usually controlled by an inverter, so that the frequency of the alternating current in the stator of the generator may be adapted to the current rotational speed of the rotor hub of the wind turbine, i.e. the wind turbine will generate alternate current at exactly the variable frequency applied to the stator.
Since alternating current with a variable frequency cannot be applied to the utility grid, it needs to be rectified or converted into direct current (DC). The conversion from variable frequency AC to DC can be executed by thyristors or transistors, for instance. Further, the direct current is reconverted to an alternating current having a frequency applicable to the utility grid. Again, either thyristors or transistors may be used to convert the direct current into alternating current. Thereby, the gained alternating current usually needs to be smoothed before it is supplied to the utility grid. Therefore, AC filter mechanism using appropriate inductances and capacitors may be used, for instance.
In order to improve the power quality of the power applied to the grid, modern wind turbines comprising a generator and a number of electrical converter units have been proposed since, by using a plurality of converter units a better control of reactive power is feasible. A typical example is disclosed in EP 1 768 223 A2 showing a wind turbine having a number of converter modules in parallel configuration.
Yet, it is a drawback of prior art using indirect grid connections, that availability rates of the wind turbines are often lower in comparison to wind turbines having a direct grid connection due to failures in the power electronics, that is mainly the electrical converter units.